Homogeneous charge compression ignition (HCCI) is a new engine combustion concept that offers a means to increase the efficiency of gasoline and diesel fueled engines. For gasoline engines, the use of HCCI combustion rather than spark ignition combustion can enable well-mixed, very lean fuel-air mixtures to be burned rapidly inside the engine cylinder at high engine compression ratio. These conditions give high engine efficiency and exceptionally low NOx,CO, and particulate emissions. However, it is extremely difficult in a practical engine concept to make gasoline HCCI combustion work satisfactorily at all loads, and in particular, at high engine loads. Load is used to denote relative torque: the ratio of actual torque to maximum torque at a given engine speed. Low load is up to about one-third maximum torque; high load is above about two-thirds maximum torque. Practical gasoline engine concepts that use the HCCI combustion mode utilize it between low- and mid-load levels, and return to the normal spark-ignition combustion mode at high loads. Such combined engine concepts thus have a severe knock problem to overcome in the high-load spark-ignition engine operating mode if the high compression ratio (up to about 15:1) desirable for the HCCI mode is used. If the compression ratio is reduced to normal values (around 10:1) to avoid this knock problem, much of the part-load HCCI efficiency benefit is lost. In addition, the onset of knock at high loads also limits the use of pressure boosting such as through turbocharging and supercharging to increase engine power density and decrease engine size.
HCCI engines can also operate on diesel fuel. High cetane diesel fuels provide the desired diesel-like performance at high loads. At low and mid-load levels, however, since the high cetane fuel is readily ignited, there is not enough time for fuel-air mixing following injection during the compression stroke to produce the desired homogeneous charge. Too rapid auto ignition at low- and mid-load levels degrades engine operation in the HCCI combustion mode. It is therefore desirable to enhance autoignition resistance at low-to mid-load levels.
Co-owned pending U.S. patent application Ser. No. 10/460,574, the contents of which are incorporated herein by reference, discloses the use of hydrogen, and hydrogen and carbon monoxide mixtures to enhance knock resistance to improve the performance of spark-ignition gasoline engines. The enhanced knock resistance is a manifestation of the increased resistance to autoignition, the self-ignition of a fuel air mixture under sufficient temperature and pressure. In particular, the pending application demonstrated that modest amounts of hydrogen addition to a gasoline vapor/air mixture within an engine cylinder can raise the fuel's octane number by ten or more numbers. When carbon monoxide is mixed with the hydrogen, with roughly equal energy content in each of these added gases, the impact can be approximately doubled (for the same hydrogen energy content). The present invention builds on the use of hydrogen and/or hydrogen and carbon monoxide mixtures to improve performance in homogeneous charge compression ignition engines.